Heat a large Dutch oven over medium-high heat. Coat pan with cooking spray.
Add onion, carrot, and celery to pan; cook 5 minutes, stirring occasionally.
While vegetables cook, heat roast beef according to package directions; drain, discarding drippings. Chop beef into 1-inch pieces.
Stir garlic into vegetables; cook 30 seconds. Stir in beef, barley, and remaining ingredients. Bring to a boil; cover, reduce heat, and simmer 40 minutes or until barley and vegetables are tender, stirring occasionally.